The great swimming speed of the blue whale, together with the remoteness of its stronghold—where three of Earth's oceans merge in the ice-cold waters around Antarctica—protected most of the species until early in the 20th century. With the invention of explosive harpoons and fast, steam-powered catcher boats, the stronghold was breached. Through the first six decades of the 20th century 360,000 blue whales were killed. The population around South Georgia Island was extirpated, along with those that once fed in the coastal waters of Japan. Some blue whale populations were reduced by ninety-nine one-hundredths, and the species tipped at the very brink of extinction.

For Bruce Mate and John Calambokidis, the head scientists aboard Pacific Storm, the irony is deep and poignant. The blue whales they study, the 2,000 animals that summer off western North America, once just a splinter group, now make up a significant population.

Mate, director of the Marine Mammal Institute at Oregon State University, is the world's most inventive and prolific satellite-tagger of whales. The dome first caught his attention in 1995, when a blue whale he had tagged off California in summer began transmitting from off Costa Rica in winter. Calambokidis, a co-founder of Cascadia Research, in Olympia, Washington, is the West Coast's most prolific photo-identifier of whales. A tall, lean biologist with a Quaker seaman's beard and monomaniacal dedication to bringing back diagnostic images, Calambo­kidis was tantalized by the reports from the satellite. In 1999 he made a reconnaissance of the dome by sailboat. The voyage was plagued by bad weather, and the sailboat was too small for its mission, yet at the dome Calambo­kidis managed to photo identify ten whales that he had photographed off California.

Why would a blue whale depart its feeding grounds at the end of summer and migrate thou­sands of miles to spend winter in this tropical zone of upwelling? Mate and Calambokidis thought they knew. The satellite data showed that some of the tagged whales lingered five months or more at the dome, arriving early in the southern migration and departing late—a pattern that, in other species of baleen whales, is seen in pregnant females and new mothers. It had never been noted in blue whales, for the best of reasons: No one has ever witnessed the birth of a blue whale.

Gray, humpback, and right whales—the baleen species that have been studied at their calving grounds—seem to feed little, if at all, at those grounds. But there is evidence that the blue whale might be different. Given its great size and enormous energy requirements, the blue whale may be forced to find winter grounds where it can do more than snack. The oasis of the Costa Rica Dome would satisfy this requirement. Plus, the productivity of the upwelling would help nursing mothers convert schools of krill into the barrels of milk required by the calves to put on their 200 pounds a day.